What kind of insect did this?

Howdy all. I am doing some work at my house and found tunnels inside several studs. No dust piles or insects, just tunnels. Some of the tunnels have what appears to be saw dust in them but it's hard. It almost looks like wood filler. The studs are all structurally sound, not soft at all. The attached picture shows an example.

Some are in load bearing walls so they are not easy to replace. I live in Michigan. The damage looks to be quite old... I think. The house itself it old - 50+ years.

1. What insect likely caused this?
2. How likely is it that the insect is still around? I haven't seen any bugs at all.

What kind of insect did this?

It is from a wood-boring larvae/beetle. Very likely been there a long time; oh'mike's advice is good. The sawdust they make is called "frass" and they use it to pack the oval opening prior to emerging. Their gestation period can be years, especially if in an unheated building. It's likely that kiln drying did kill them and no re-infestation took place. Oftentimes they don't re-infest anyway due to the wood changing it's moisture and nutrient content over time.
They do make oval shaped holes though. The longer galleries indicate that the milling process exposed them, as oh'mike points out.